RFC9612: Bidirectional Forwarding Detection (BFD) Reverse Path for MPLS Label Switched Paths (LSPs)
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(BFD)
(lsp ping)
Internet Engineering Task Force (IETF) G. Mirsky
Request for Comments: 9612 Ericsson
Category: Experimental J. Tantsura
ISSN: 2070-1721 NVIDIA
I. Varlashkin
Google
M. Chen
Huawei
July 2024
Bidirectional Forwarding Detection (BFD) Reverse Path for MPLS Label
Switched Paths (LSPs)
Abstract
Bidirectional Forwarding Detection (BFD) is expected to be able to
monitor a wide variety of encapsulations of paths between systems.
When a BFD session monitors an explicitly routed unidirectional path,
there may be a need to direct the egress BFD peer to use a specific
path for the reverse direction of the BFD session. This document
describes an extension to the MPLS Label Switched Path (LSP) echo
request that allows a BFD system to request that the remote BFD peer
transmit BFD control packets over the specified LSP.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9612.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Conventions Used in This document
1.1.1. Terminology
1.1.2. Requirements Language
2. Problem Statement
3. Control of the BFD Reverse Path
3.1. BFD Reverse Path TLV
3.2. Return Codes
3.3. Failure Characterization
4. Use Case Scenario
5. Operational Considerations
6. IANA Considerations
6.1. BFD Reverse Path TLV
6.2. Return Codes
7. Security Considerations
8. Normative References
Acknowledgments
Authors' Addresses
1. Introduction
[RFC5880], [RFC5881], and [RFC5883] established the Bidirectional
Forwarding Detection (BFD) protocol for IP networks. [RFC5884] and
[RFC7726] set rules for using BFD Asynchronous mode over MPLS Label
Switched Paths (LSPs), while not defining means to control the path
that an egress BFD system uses to send BFD control packets towards
the ingress BFD system.
When BFD is used to detect defects of the traffic-engineered LSP, the
path of the BFD control packets transmitted by the egress BFD system
toward the ingress may be disjoint from the monitored LSP in the
forward direction. The fact that BFD control packets are not
guaranteed to follow the same links and nodes in both forward and
reverse directions may be one of the factors contributing to false
positive defect notifications (i.e., false alarms) at the ingress BFD
peer. Ensuring that both directions of the BFD session use co-routed
paths may, in some environments, improve the determinism of the
failure detection and localization.
This document defines the BFD Reverse Path TLV as an extension to LSP
ping [RFC8029] and proposes that it be used to instruct the egress
BFD system to use an explicit path for its BFD control packets
associated with a particular BFD session. IANA has registered this
TLV in the "TLVs" registry defined by [RFC8029] (see Section 6.1).
As a special case, forward and reverse directions of the BFD session
can form a bidirectional co-routed associated channel.
The LSP ping extension described in this document was developed and
implemented as a result of an operational experiment. The lessons
learned from the operational experiment enabled the use of this
extension between systems conforming to this specification. Further
implementation is encouraged to better understand the operational
impact of the mechanism described in the document.
1.1. Conventions Used in This document
1.1.1. Terminology
BFD: Bidirectional Forwarding Detection
FEC: Forwarding Equivalence Class
LSP: Label Switched Path
LSR: Label Switching Router
1.1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Problem Statement
When BFD is used to monitor an explicitly routed unidirectional path
(e.g., MPLS-TE LSP), BFD control packets in the forward direction
would be in-band using the mechanism defined in [RFC5884]. However,
the reverse direction of the BFD session would follow the shortest
path route, which could be completely or partially disjoint from the
forward path. This creates the potential for the failure of a
disjoint resource on the reverse path to trigger a BFD failure
detection, even though the forward path is unaffected.
If the reverse path is congruent with the forward path, the potential
for such false positives is greatly reduced. For this purpose, this
specification provides a means for the egress BFD peer to be
instructed to use a specific path for BFD control packets.
3. Control of the BFD Reverse Path
To bootstrap a BFD session over an MPLS LSP, LSP ping [RFC8029] MUST
be used with the BFD Discriminator TLV [RFC5884]. This document
defines a new TLV, the BFD Reverse Path TLV, that can be used to
carry information about the reverse path for the BFD session that is
specified by the value in the BFD Discriminator TLV. The BFD Reverse
Path TLV MAY contain zero or more sub-TLVs.
3.1. BFD Reverse Path TLV
The BFD Reverse Path TLV is an optional TLV within the LSP ping
[RFC8029]. However, if used, the BFD Discriminator TLV MUST be
included in an echo request message as well. If the BFD
Discriminator TLV is not present when the BFD Reverse Path TLV is
included, then it MUST be treated as a malformed echo request, as
described in [RFC8029].
The BFD Reverse Path TLV carries information about the path onto
which the egress BFD peer of the BFD session referenced by the BFD
Discriminator TLV MUST transmit BFD control packets. The format of
the BFD Reverse Path TLV is presented in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BFD Reverse Path TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Path |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: BFD Reverse Path TLV
BFD Reverse Path TLV Type:
This two-octet field has a value of 16384 (see Section 6).
Length:
This two-octet field defines the length in octets of the Reverse
Path field.
Reverse Path:
This field contains zero or more sub-TLVs. Only non-multicast
Target FEC Stack sub-TLVs (already defined or to be defined in the
future) for TLV Types 1, 16, and 21 in the "Multiprotocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry are permitted to be used in this field. Other sub-TLVs
MUST NOT be used. (This implies that multicast Target FEC Stack
sub-TLVs, e.g., the Multicast P2MP LDP FEC Stack sub-TLV and the
Multicast MP2MP LDP FEC Stack sub-TLV, are not permitted in the
Reverse Path field.)
If the egress LSR finds a multicast Target FEC Stack sub-TLV, it MUST
send an echo reply with the received BFD Reverse Path TLV and BFD
Discriminator TLV and set the Return Code to 192 ("Inappropriate
Target FEC Stack sub-TLV present") (see Section 3.2). The BFD
Reverse Path TLV includes zero or more sub-TLVs. However, the number
of sub-TLVs in the Reverse Path field MUST be limited. The default
limit is 128 sub-TLV entries, but an implementation MAY be able to
control that limit. An empty BFD Reverse Path TLV (i.e., a BFD
Reverse Path TLV with no sub-TLVs) is used to withdraw any previously
set reverse path for the BFD session identified in the BFD
Discriminator TLV. If no sub-TLVs are found in the BFD Reverse Path
TLV, the egress BFD peer MUST revert to using the decision based on
local policy, i.e., routing over an IP network, as described in
Section 7 of [RFC5884].
If the egress peer LSR cannot find the path specified in the BFD
Reverse Path TLV, it MUST send an echo reply with the received BFD
Discriminator TLV and BFD Reverse Path TLV and set the Return Code to
193 ("Failed to establish the BFD session. The specified reverse
path was not found.") (see Section 3.2). If an implementation
provides additional configuration options, these can control actions
at the egress BFD peer, including when the path specified in the BFD
Reverse Path TLV cannot be found. For example, if the egress peer
LSR cannot find the path specified in the BFD Reverse Path TLV, it
MAY establish the BFD session over an IP network, as defined in
[RFC5884]. Note that the Return Code required by the "MUST" clause
in this paragraph does not preclude the session from being
established over a different path as discussed in the "MAY" clause.
The BFD Reverse Path TLV MAY be used in the process of bootstrapping
the BFD session as described in Section 6 of [RFC5884]. A system
that supports this specification MUST support using the BFD Reverse
Path TLV after the BFD session has been established. If a system
that supports this specification receives an LSP ping with the BFD
Discriminator TLV and no BFD Reverse Path TLV even though the reverse
path for the specified BFD session was established according to the
previously received BFD Reverse Path TLV, the egress BFD peer MUST
transition to transmitting periodic BFD Control messages as described
in Section 7 of [RFC5884]. If a BFD system that received an LSP ping
with the BFD Reverse Path TLV does not support this specification, it
will result in an echo response with the Return Code set to 2 ("One
or more of the TLVs was not understood"), as described in Section 3
of [RFC8029].
3.2. Return Codes
This document defines the following Return Codes for the MPLS LSP
echo reply:
"Inappropriate Target FEC Stack sub-TLV present" (192):
When a multicast Target FEC Stack sub-TLV is found in the received
echo request, the egress BFD peer sends an echo reply with the
Return Code set to 192 ("Inappropriate Target FEC Stack sub-TLV
present") to the ingress BFD peer, as described in Section 3.1.
"Failed to establish the BFD session. The specified reverse path
was not found." (193):
When a specified reverse path is unavailable, the egress BFD peer
sends an echo reply with the Return Code set to 193 ("Failed to
establish the BFD session. The specified reverse path was not
found.") to the ingress BFD peer, as described in Section 3.1.
3.3. Failure Characterization
A failure detected by a BFD session that uses the BFD Reverse Path
TLV could be due to a change in the FEC used in the BFD Reverse Path
TLV. Upon detection of the network failure, the ingress BFD peer
MUST transmit the LSP ping echo request with the Reply Path TLV
[RFC7110] to verify whether the FEC is still valid. If the failure
was caused by a change in the FEC used for the reverse direction of
the BFD session, the ingress BFD peer MUST redirect the reverse path
of the BFD session using another FEC in the BFD Reverse Path TLV and
notify an operator.
4. Use Case Scenario
In the network presented in Figure 2, ingress LSR peer A monitors two
tunnels to egress LSR peer H: A-B-C-D-G-H and A-B-E-F-G-H. To
bootstrap a BFD session to monitor the first tunnel, ingress LSR peer
A includes a BFD Discriminator TLV with a Discriminator value (e.g.,
foobar-1) [RFC7726]. Ingress LSR peer A includes a BFD Reverse Path
TLV referencing the H-G-D-C-B-A tunnel to control the path from the
egress LSR. To bootstrap a BFD session to monitor the second tunnel,
ingress LSR peer A includes a BFD Discriminator TLV with a different
Discriminator value (e.g., foobar-2) and a BFD Reverse Path TLV that
references the H-G-F-E-B-A tunnel.
C---------D
| |
A-------B G-----H
| |
E---------F
Figure 2: Use Case for BFD Reverse Path TLV
If an operator needs egress LSR peer H to monitor a path to ingress
LSR peer A, e.g., the H-G-D-C-B-A tunnel, then by looking up the list
of known reverse paths, it MAY find and use the existing BFD session.
5. Operational Considerations
When an explicit path is set as either Static or RSVP-TE LSP,
corresponding sub-TLVs (defined in [RFC7110]) MAY be used to identify
the explicit reverse path for the BFD session. If a particular set
of sub-TLVs composes the Reply Path TLV [RFC7110] and does not
increase the length of the Maximum Transmission Unit for the given
LSP, that set can be safely used in the BFD Reverse Path TLV. If any
of the sub-TLVs defined in [RFC7110] are used in the BFD Reverse Path
TLV, then the periodic verification of the control plane against the
data plane, as recommended in Section 4 of [RFC5884], MUST use the
Reply Path TLV, as per [RFC7110], with that sub-TLV. By using the
LSP ping with the Reply Path TLV, an operator monitors whether the
reverse LSP is mapped to the same FEC as the BFD session at the
egress BFD node. Selection and control of the rate of the LSP ping
with the Reply Path TLV follows the recommendation in [RFC5884]:
| The rate of generation of these LSP Ping Echo request messages
| SHOULD be significantly less than the rate of generation of the
| BFD Control packets. An implementation MAY provide configuration
| options to control the rate of generation of the periodic LSP Ping
| Echo request messages.
Suppose an operator planned a network maintenance activity that
possibly affects the FEC used in the BFD Reverse Path TLV. In that
case, the operator can avoid unnecessary disruption by using the LSP
ping with a new FEC in the BFD Reverse Path TLV. But in some
scenarios, proactive measures cannot be taken because the frequency
of LSP ping messages is lower than the defect detection time provided
by the BFD session. As a result, a change in the reverse-path FEC
will first be detected as the BFD session's failure. An operator
will be notified as described in Section 3.3.
6. IANA Considerations
6.1. BFD Reverse Path TLV
IANA has assigned the following value for the BFD Reverse Path TLV
from the 16384-31739 range in the "TLVs" subregistry within the
"Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
Ping Parameters" registry.
+=======+=========+===========+====================================+
| Type | TLV | Reference | Sub-TLV Registry |
| | Name | | |
+=======+=========+===========+====================================+
| 16384 | BFD | RFC 9612 | Only non-multicast sub-TLVs |
| | Reverse | | (already defined or to be defined |
| | Path | | in the future) in the "Sub-TLVs |
| | | | for TLV Types 1, 16, and 21" |
| | | | registry at |
| | | | <https://www.iana.org/assignments/ |
| | | | mpls-lsp-ping-parameters/mpls-lsp- |
| | | | ping-parameters.xml#sub-tlv- |
| | | | 1-16-21> are permitted to be used |
| | | | in this field. Other sub-TLVs |
| | | | MUST NOT be used. |
+-------+---------+-----------+------------------------------------+
Table 1: New BFD Reverse Path TLV
6.2. Return Codes
IANA has assigned the following Return Code values from the 192-247
range in the "Return Codes" subregistry within the "Multiprotocol
Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
registry.
+=======+===========================================+===========+
| Value | Meaning | Reference |
+=======+===========================================+===========+
| 192 | Inappropriate Target FEC Stack sub-TLV | RFC 9612 |
| | present | |
+-------+-------------------------------------------+-----------+
| 193 | Failed to establish the BFD session. The | RFC 9612 |
| | specified reverse path was not found. | |
+-------+-------------------------------------------+-----------+
Table 2: New Return Codes
7. Security Considerations
Security considerations discussed in [RFC5880], [RFC5884], [RFC7726],
[RFC8029], and [RFC7110] apply to this document.
The BFD Reverse Path TLV may be exploited as an attack vector by
inflating the number of included sub-TLVs. The number of sub-TLVs
MUST be limited to mitigate that threat. The default limit for the
number of sub-TLVs is set to 128 (see Section 3.1). An
implementation MAY use a mechanism to control that limit.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://www.rfc-editor.org/info/rfc5881>.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
June 2010, <https://www.rfc-editor.org/info/rfc5883>.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
June 2010, <https://www.rfc-editor.org/info/rfc5884>.
[RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
"Return Path Specified Label Switched Path (LSP) Ping",
RFC 7110, DOI 10.17487/RFC7110, January 2014,
<https://www.rfc-editor.org/info/rfc7110>.
[RFC7726] Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S.
Aldrin, "Clarifying Procedures for Establishing BFD
Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726,
DOI 10.17487/RFC7726, January 2016,
<https://www.rfc-editor.org/info/rfc7726>.
[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<https://www.rfc-editor.org/info/rfc8029>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Acknowledgments
The authors greatly appreciate the thorough reviews and helpful
comments from Eric Gray and Carlos Pignataro. The authors much
appreciate the help of Qian Xin, who provided information about the
implementation of this specification.
Authors' Addresses
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Jeff Tantsura
NVIDIA
Email: jefftant.ietf@gmail.com
Ilya Varlashkin
Google
Email: imv@google.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com